Slate's Timothy Noah has put together a ten-part series on inequality, called "The Great Divergence." It's terrific, based on what I've seen so far. And it includes this very telling graph, using figures from political scientist Larry Bartels:

Noah explains:

Did the United States grow more unequal while Republicans where in power? It sounds crude, but Princeton political scientist Larry Bartels has gone a long way toward proving it. Bartels looked up income growth rates for families at various income percentiles for the years 1948 to 2005, then cross-checked these with whether the president was a Republican or Democrat. He found two distinct and opposite trends. Under Democrats, the biggest income gains were for people in the bottom 20th income percentile ... The income games grew progressively smaller further up the income scale ... But under Republicans, the biggest income gains were for people in the 95th percentile ... The income gains grew progressively smaller further down the income scale.